Known in the art is a spark ignition type internal combustion engine provided with a variable compression ratio mechanism able to change a mechanical compression ratio and a variable valve mechanism able to control a closing timing of an intake valve, performing a supercharging action by a supercharger at the time of engine medium load operation and engine high load operation, and increasing the mechanical compression ratio and retarding the closing timing of the intake valve as the engine load becomes lower in the state holding an actual compression ratio constant at the time of engine medium and high load operation (for example, see Japanese Patent Publication (A) No. 2004-218522).
According to such a spark ignition type internal combustion engine, at the time of engine medium load operation, the mechanical compression ratio rises and the closing timing of the intake valve is retarded, so the fuel consumption at the time of engine medium load operation improves, whereas at the time of engine high load operation, the mechanical compression ratio drops and the closing timing of the intake valve is advanced, so the output torque at the time of engine high load operation increases.
However, in such a spark ignition type internal combustion engine, the variable compression ratio mechanism and variable valve mechanism are controlled in a way that keeps the actual compression ratio constant. Specifically, a closing timing of the intake valve is determined to make the intake air amount correspond to the amount of engine load, and then a mechanical compression ratio is determined based on the determined closing timing of the intake valve to make the actual compression ratio constant.
However, such a control method cannot sufficiently improve thermal efficiency. That is, in order to increase the thermal efficiency, it is necessary to increase the mechanical expansion ratio (equivalent to the mechanical compression ratio) as much as possible. However, under the above control method, the mechanical expansion ratio is determined in a dependent manner in accordance with the closing timing etc. of the intake valve determined first, so the mechanical expansion ratio is not necessarily set as much as possible.